CERN-LHC. A measurement of the total $pp$ cross section at the LHC at $\sqrt{s}=8$ TeV is presented.
An integrated luminosity of $500$ $\mu$b$^{-1}$ was accumulated
in a special run with high-$\beta^{\star}$ beam optics
to measure the differential elastic cross section as a function of
the Mandelstam momentum transfer variable $t$. The measurement is performed with the ALFA sub-detector of
ATLAS. Using a fit to the differential elastic cross section in the $-t$ range from $0.014$ GeV$^2$ to $0.1$ GeV$^2$ to
extrapolate $t\rightarrow 0$, the total cross section, $\sigma_{\mathrm{tot}}(pp\rightarrow X)$,
is measured via the optical theorem to be:
$\sigma_{\mathrm{tot}}(pp\rightarrow X) = \mbox{96.07} \; \pm 0.18 \; ({\mbox{stat.}}) \pm 0.85 \;
({\mbox{exp.}}) \pm 0.31 \; (\mbox{extr.}) \; \mbox{mb} \;,$
where the first error is statistical, the second accounts for all experimental systematic uncertainties and the last
is related to uncertainties in the extrapolation $t\rightarrow 0$.
In addition, the slope of the exponential function describing the elastic cross section at small $t$ is determined to be $B = 19.74 \pm 0.05 \; ({\mbox{stat.}}) \pm 0.23 \; ({\mbox{syst.}}) \; \mbox{GeV}^{-2}$.

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